Many diffractive optics devices, such as those used for focusing X-rays or particle rays (elementary particles or ions), use high-aspect-ratio or free-standing curved or variable-width line grids, or, they are composed of a succession of absorption or refractive index varying absorber or phase shifter materials to manipulate the wave front through superposition of beams diffracted by individual design features of the devices. In many cases, such design features are organized in curved or straight lines of variable spacing. The phrase “diffractive lines” will be used herein to describe the diffractive elements of such devices. For some geometries, the diffractive lines may alternatively be referred to as diffractive zones. A typical example of this type of structure is a Fresnel Zone Plate (FZP) for focusing X-rays, in which the diffractive lines are organized in concentric circles, which may be referred to as zones. The width of the absorber/phase shifter material lines (zones) is varied in proportion to the square root of the radius of curvature of the lines, usually from widths of a few micrometers at the center of the FZP, to a few tens of nanometers or less at the outermost zones. The characteristic performance metrics of such devices, such as focusing resolution, diffraction efficiency, spectral resolution, or other metrics, depend on the linewidth control and fidelity of the smallest dimension diffractive lines/zones and the thickness in the propagation direction of the diffractive lines, as achievable technologically. For instance, the focusing resolution of FZPs is related to the width of the outermost circular zone, while the efficiency—defined as the energy of the beam diffracted towards the focus of interest divided by the total incident beam energy—depends on the thickness and nature of the material, as well as the accuracy of the zone pattern. It is well-known to those skilled in the art that the maximum theoretical efficiency of binary FZPs can be obtained if the phase shift difference in adjacent zones is a multiple of pi (π). For high photon energy X-rays such as 25 keV, the necessary thickness of a π-shifting material is about 4.84 μm for gold, 4.5 μm for tungsten, and 4.2 μm for iridium. If the smallest zone width is 20 nm, then the aspect ratios of such diffractive lines are 242:1, 225:1, and 210:1, respectively. Self-standing geometric objects of this aspect ratio are essentially impossible to fabricate without collapse or distortion at this time. If a material scaffold of low absorption/low refractive index is used to prevent collapse, the phase shift produced in that material has to be considered and the aspect ratio needed for a phase shift difference of π between adjacent diffractive lines increases even more. Also, if applications are envisioned for higher photon energies, such as 20-100 keV, where X-ray producing tubes/lamps generate higher intensities due to the energy proximity of spectral absorption/emission edges of most metallic cathode electrodes of interest (including the refractory metals well-suited for X-ray emission cathodes), the aspect ratio requirements of the corresponding diffractive lines increases dramatically up to tens of thousands-to-one. The use of small laboratory-based X-ray sources such as X-ray tubes is preferable to sources such as those of expensive synchrotron radiation facilities, to allow for lower cost/smaller footprint generation of X-rays in small X-ray microscopes or diffraction devices, which then require adequate diffraction optics of super-high aspect ratios. Recently, D. Habs et al. (D. Habs et al., Phys. Rev. Lett. 108, (2012) 184802) demonstrated that the refractive indexes of materials in the gamma (γ)-ray regime (photon energies from 0.18 MeV to 2 MeV) can attain values in the 10−9-10−5 range due to inelastic Delbrück scattering or pair creation, thus allowing for the possibility of the fabrication of diffractive optics for γ-rays, if super-high aspect ratio structures can be reliably fabricated. Thus, there exists a need for a method of greatly improving the manufacturability of high aspect-ratio diffractive optics devices comprised of appropriate phase shifting materials which are capable of addressing one or more limitations of known methods.
Several methods of manufacturing diffractive optics of high aspect ratio have been proposed and practiced. The most common manufacturing method of limited high aspect ratio diffractive optics is the use of electron beam lithography for writing the diffractive lines into an electron-beam-sensitive resist material, followed by electroforming a metal using the said resist as a mold, as taught by B. Lai et al. (B. Lai et al., Appl. Phys. Lett. 61, 1877 (1992)). Electron beam lithography followed by the etching of the substrate and filling the so-obtained mold with a metal by electroplating is also known, as taught, for example, by A. Stein et al. for the case of etching the structure into silicon (A. Stein et al., J. Vac. Sci. Technol. B 21.1., January/February 2003, 214-219), or K. Jefimov et al. for the case of polyimide (K. Jefimov et al., Microelectronic Engineering 84 (2007) 1467-1470), or J. Reinspach et al. for germanium (J. Reinspach et al., J. Vac. Sci. Technol. B, Vol. 29, No. 1, January/February 2011, 011012-1-011012-4), or C. David et al. for diamond (C. David et al., Scientific Reports, (2011) 1: 57). Electron beam lithography can be replaced by other types of lithography, such as, X-ray, ion beam, focused ion beam or particle beam lithography, followed by etching and/or electroplating (W. Yun et al., Rev. of Sci. Instruments 70, 5, (1999), 2238-2241; and K. Keskinbora et al., Optics Express, Vol. 21, No. 10 (2013) 11747-11756). The limitations of all these types of lithography and methods are the modest aspect ratios of the features that they are capable of producing. These prior art methods usually achieve an aspect ratio of 10:1, and only a few exceptions can reach aspect ratios close to about 20:1. These aspect ratios are insufficient for efficient focusing of x-rays, or particle beams of a corresponding energy.
To overcome the aspect ratio limitation in various lithographic technologies, a method of producing diffractive optics of circular symmetry, called “sputter-and-slice” was proposed by K. Saitoh et al. (K. Saitoh et al., Rev. of Sci. Instruments 60, 7, (1989), 1519-1523). The method begins with a wire, rod, or tube, of circular cross-section, followed by deposition of a succession of alternating layers of material having different characteristics, i.e., alternating low absorbing/high absorbing material layers, or, alternating low refractive index/high refractive index material layers. Layers are deposited by sputtering from different targets in the same vacuum chamber, with rotation of the wire around its axis. The deposition times and conditions are calculated such that the thicknesses of the successively deposited layers are equal to the diffractive line width of a given position in the targeted device. Finally, the wire is sectioned and polished to form the diffractive device, such as, a Fresnel zone plate. One drawback of this method is the accentuation or amplification of radius irregularities of the central wire (i.e. the roughness or accidental other variations including particulate defects and thickness variations during the deposition) as successive layers are deposited beginning from the bare wire and proceeding to the outermost layer. A second drawback is the accumulation of the absolute error values of the depositions thicknesses of each of the inward layers as an error of placement of the next outward layer. Thus, the outermost (i.e. thinnest) layers accumulate the largest radius errors due to the amplification and accumulation of the depositional processes errors through successive layers. This error amplification process can produce defects in the final (thinnest) zone shape and placement exceeding a half-width of the layer/zone itself, rendering that portion of the diffractive line useless for focusing or adequate manipulation of phase shift of the prescribed photon or particle beam. Improvements of the sputter-and-slice method have been proposed by M. Yasumoto et al., (M. Yasumoto et al., Japanese Journal of Applied Physics, 40 (2001), pp. 4747-4748), in which the sputtering of layers onto the rotating wire is performed through a narrow slit, to minimize the roughness accentuating phenomenon. However, in order to effectively reduce the error amplification process, the slit widths have to be reduced to fractions of the wire diameter, which reduces the deposition rate to impractically low values. Another drawback of the sputter-and-slice method is that only one wire can be processed at a time, while the slicing/polishing procedures are serial and time-consuming.
A linear form of the sputter-and-slice method has been developed for one dimensional focusing lenses, in which a planar substrate is sputter-deposited with successive layers of designed thickness by moving the planar substrate below sputtering targets and then slicing and using the slices grouped in a symmetrical pair as linear focusing lenses, called Multi-Layer Laue Lenses (MLLs) (H. Yan, et al., Optics express Vol. 19, No. 16, (2011), 15069-15076). The method suffers from the same drawbacks as the wire-based version and can be used only for linear (one-dimensional) optics; however, it has shown relatively higher rates of success because it is easier to obtain highly planar and smoothly-polished planar surfaces rather than perfectly circular, low roughness wires or tubes.
An alternative version of the sputter-and-slice method has been developed in which the directional sputtering process onto wires is replaced by atomic layer deposition (ALD), onto wires. ALD is by nature, a highly isotropic deposition process (see review by S. M. George, Chem. Rev., 110 (1), (2010), pp 111-131). The method, reported by M. Mayer et al. (M. Mayer et al., Ultramicroscopy 111 (2011) 1706-1711), which is known as “ALD-and-slice”, eliminates the roughness accentuation of the sputter-and-slice method and even produces a smoothing with succeeding depositions, does not require a rotation setup in the deposition chamber, and permits deposition onto several wires at the same time. However, this method still preserves error accumulation in layer thicknesses, but these errors are more easily kept in control, due to the more precise nature of the ALD process. A version of the ALD-and-slice method, in which successive layers are deposited into the inner cavity of a capillary tube, rather than onto a wire, was proposed by G. Schuetz et al., (U.S. patent application No. 2012/0258243 A1). This method alleviates the problem of error accumulation in the thinnest (top most) layers, since the thicker layers are deposited last, when the accumulated error is higher, but those thicker layers are also more shape and placement tolerant (in absolute values), thus, diminishing the percentage of thickness of accumulated error in the last (thicker) layers/zones. All the versions of the sputter and slice and ALD-and-slice methods preserve the drawbacks of a serial and tedious slicing and polishing procedure. An additional drawback of the ALD-and-slice method is the relative slow rate of deposition in ALD processes, which require months-long deposition times for Fresnel Zone Plates with a reasonable number of zones.
A related method was proposed by W. Yun et al. (U.S. Pat. No. 7,365,918 B1) comprising etching a cylindrical hole into a substrate and sequentially depositing layers corresponding in thickness to zone plate zones, by sputtering or ALD, and then slicing by polishing to form zone plates. In practice, for deposition by sputtering, this fabrication process would be limited by the variation of thickness of sputtered layers with the depth in the hole, inherent to the sputtering process. As mentioned above, while sequential ALD is capable of deposition of conformal layers of uniform and controlled thickness, in practice, the method of Yun et al. would be limited by the surface roughness of the cylindrical holes, which depends on the etch process used for etching the holes. For example, a deep reactive ion etching process (DRIE), such as the Bosch etch process, which is used to etch high aspect-ratio, deep, holes for microelectronics or MEMS devices, is known to create sidewall ripples, or “scalloping” of the sidewalls. Thus, the thickness of the thinnest functional zone of the diffractive optic is limited by the average roughness of the side walls, which is usually tens or hundreds of nm for most etching processes. Moreover, this method would need an accurate characterization of the diameter, sidewall tilt and surface roughness of the cylindrical hole, e.g., an accuracy of no less than half of the thinnest targeted functional zone width, which is typically ˜10 nm or less. This accuracy is difficult to characterize in holes of the required depth and aspect ratio, except by destructive procedures.
Thus there is a need for improved device structures and methods for fabrication of high aspect ratio diffractive optics, which address one or more of the above mentioned limitations of known device structures and methods.